导入Sanic程序至其他程序时触发‘context has already been set’错误求助

环境信息 #

• Python版本：3.10
• Sanic-Cors版本：2.2.0
• Sanic版本：24.6.0
• 操作系统：Windows 11

问题现象 #

直接运行api.py无任何错误，但运行main.py调用启动函数时触发RuntimeError，错误日志如下：

Main  2024-08-15 21:58:44 +0800 INFO: Sanic v24.6.0
Main  2024-08-15 21:58:44 +0800 INFO: Goin' Fast @ http://127.0.0.1:5050
Main  2024-08-15 21:58:44 +0800 INFO: app: ip
Main  2024-08-15 21:58:44 +0800 INFO: mode: production, single worker
Main  2024-08-15 21:58:44 +0800 INFO: server: sanic, HTTP/1.1
Main  2024-08-15 21:58:44 +0800 INFO: python: 3.12.4
Main  2024-08-15 21:58:44 +0800 INFO: platform: Windows-11-10.0.22631-SP0
Main  2024-08-15 21:58:44 +0800 INFO: packages: sanic-routing==23.12.0
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "xxx\Python\Python312\Lib\multiprocessing\spawn.py", line 122, in spawn_main
    exitcode = _main(fd, parent_sentinel)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "xxx\Python\Python312\Lib\multiprocessing\spawn.py", line 131, in _main
    prepare(preparation_data)
  File "xxx\Python\Python312\Lib\multiprocessing\spawn.py", line 246, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "xxx \Python\Python312\Lib\multiprocessing\spawn.py", line 297, in _fixup_main_from_path
    main_content = runpy.run_path(main_path,
                   ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen runpy>", line 286, in run_path
  File "<frozen runpy>", line 98, in _run_module_code
  File "<frozen runpy>", line 88, in _run_code
  File "xxx\test\main.py", line 3, in <module>
    api.start_server()
  File "xxx\test\api.py", line 16, in start_server
    app.run(host="127.0.0.1", port=5050)
  File "xxx\Python\Python312\Lib\site-packages\sanic\mixins\startup.py", line 291, in run
    serve(primary=self)  # type: ignore
    ^^^^^^^^^^^^^^^^^^^
  File "xxx\Python\Python312\Lib\site-packages\sanic\mixins\startup.py", line 1003, in serve
    cls._set_startup_method()
  File "xxx\Python\Python312\Lib\site-packages\sanic\mixins\startup.py", line 932, in _set_startup_method
    set_start_method(method, force=cls.test_mode)
  File "xxx\Python\Python312\Lib\multiprocessing\context.py", line 247, in set_start_method
    raise RuntimeError('context has already been set')
RuntimeError: context has already been set

相关代码 #

api.py #

# api.py
from sanic import Sanic, json
from sanic_cors import CORS

app = Sanic("ip")
CORS(app)


@app.route("/get_proxy")
async def ip_api(request):
    ip = "XXXXXX"
    return json({"ip": ip})


def start_server():
    app.run(host="127.0.0.1", port=5050)


if __name__ == '__main__':
    start_server()

main.py #

# main.py
import api

api.start_server()

解决方案 #

原因分析 #

Windows系统下Sanic默认使用spawn方式启动子进程，当通过main.py导入api模块时，主模块（main.py）会被子进程重新导入执行，导致多进程上下文被重复设置，触发RuntimeError。

解决方法 #

方法1：修改main.py，将启动代码放入模块主入口块

修改后的main.py：

# main.py
import api

if __name__ == '__main__':
    api.start_server()

这种方式符合Python多进程编程规范，能避免子进程重复执行启动逻辑，从根源上解决上下文重复设置的问题。

方法2：指定Sanic以单进程模式启动

修改api.py中的start_server函数，显式指定workers=1：

def start_server():
    app.run(host="127.0.0.1", port=5050, workers=1)

单进程模式下Sanic不会创建子进程，也就不会触发多进程上下文冲突，但这种方式会限制服务的并发能力，仅适合测试场景或低并发需求。

内容的提问来源于stack exchange，提问作者Autopy